Small RNAs play remarkable roles in regulating gene expression by binding to Argonaute family proteins and guiding them to recognize their targets. A specific subfamily of Argonaute, named PIWI, binds PIWI-interacting RNA (piRNA) and promotes fertility in all animals tested to date. piRNAs and PIWI proteins are highly enriched in germ cells, and PIWI mutations lead to sterility and dramatically reduced germ-cell numbers. In addition, PIWI-like proteins are overexpressed in several cancer tissues and this overexpression has been linked to poor prognosis. However, the biogenesis of piRNAs, as well as the mechanism by which piRNAs regulate germ cell and cancer cell, remain largely unknown. In this proposal I will use specific experiments to address three fundamental questions about piRNA pathways using nematode C. elegans as model organism: What is the molecular mechanism of piRNA-induced gene silencing? What are the genes regulated by piRNAs? Finally, what are the components involved in piRNA biogenesis and downstream silencing? The understanding of this evolutionally conserved pathway has potential to provide novel therapeutic approaches for treatment of infertility and certain cancers.